The present invention relates in general to a D-A (Digital-to-Analog) converting circuit and more particularly to a PCM encoder of successive comparison type in which the D-A converting circuit is applied to a local decoder. Specifically, the present invention concerns a .mu.-law PCM encoder.
Remarkable progress in the field of the semiconductor technology in these years makes it possible to implement circuit elements in the form of LSI circuits with significantly improved precision and greatly enhanced performance, which in turn has motivated vigorous activity for developing LSI circuits for communication purposes with a view to realizing cheaper communication systems imparted with improved performances in the field of the communication technology.
The PCM encoder on which severe requirements are imposed in respect of the accuracy of the encoding operation has heretofore been constructed by using circuit elements or parts carefully selected and objectives for economization have been made by processing a large number of lines or channels on the basis of time-sharing multiplex techniques by using a single PCM encoder unit which is capable of operating at a high speed. However, if the PCM encoder can be implemented in the form of a LSI circuit, a so-called single channel coding system in which a PCM encoder is installed for every voice channel will become practical, since the PCM encoder device is then realized inexpensively in a small size. In this case, the PCM encoders are not required to be operated at a high speed in contrast to the prior art system where high speed is required due to the time-sharing multiplex operation. The PCM encoder is rather allowed to be operated at a relatively low speed suited for the LSI configuration to accomplish the encoding purpose.
By the way, according to the .mu.-law PCM encoding stipulated in Rec. G711 of CCITT (International Telegram and Telephone Consultive Committee), a voice signal has to be converted into a 8-bit code (where one bit represents the polarity of the signal) on the basis of a compressing law according to which the characteristic of .mu.=255 is approximated with 15 segments or chords. The compressed quantizing characteristic is then such that the negative and positive polarity portions of the characteristic are divided into 8 segments or chords I, . . . , VIII, respectively, with each of the segments being divided into 16 steps, as is illustrated in FIG. 1.
In this regard, it will be noted that the first segment I is divided into 151/2 steps because of the quantization of a mid-tread and differs from the other segments II, . . . , VIII in this respect, as can be seen from FIG. 2 which is a partial enlarged view of FIG. 1. Further, it is to be noted that, when two adjacent segments are concerned, the step in the segment distant from the origin is twice as large as the step in the segment or chord near the origin. Consequently, the boundary values among the individual segments of the .mu.-law quantizing characteristic curve are the odd numbers 31, 95, 223, 479, 991, 2015 and 4063.
The PCM encoder is generally of successive comparison type in which use is made of charge redistribution in a binary weighted capacitor array. Particularly, when the encoding according to the .mu.-law is to be effected, the peculiarity of the first segment in respect of the number of steps described above makes difficult the implementation of the encoder circuit. For example, in the case of a capacitor array which includes eight capacitors having capacitances in ratio of 2.sup.0, 2.sup.1, 2.sup.2, . . . 2.sup.7 each having one end connected in common to one input line of a voltage comparator while the other ends of the individual capacitors can be changed-over to the ground potential or a reference potential through respective switches, the output voltages derived from the capacitor array in dependence on the combinations of the change-over switches correspond to multiples of "2". Accordingly, the capacitor array used to determine the segment for the .mu.-law PCM encoding will not allow the boundary values in odd numbers to be derived without difficulty.
A PCM encoder of successive comparison type in which the segments are determined with the aid of a capacitor array composed of eight binary-weighted capacitors with the steps being determined by a resistor string is reported in IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. SC-14, No. 1, Feb. 1979, pp. 65-73.
In the case of the encoder disclosed in the literature cited above, one end of each of the eight binary-weighted capacitors are connected in common to an input terminal of a comparator, while the other ends of the capacitors are adapted to be selectively connected to voltage terminals X, Y and Z through switch means. The terminal X serves as both a sample voltage input terminal and a ground potential supply terminal, the terminal Y is supplied with positive and negative reference voltages, and finally the terminal Z is supplied with step voltages derived through division of the reference voltages by the resistor string. In this encoder arrangement, the individual capacitors are first charged with the sample voltage and subsequently the switches of the capacitor array and taps for extracting the step voltages from the resistor string are successively changed over to thereby obtain a 8-bit code corresponding to the sample voltage.
The procedure heretofore adopted for effecting the encoding according to the .mu.-law with fidelity by using the encoder of the above-mentioned type resides in that each of the positive and negative reference voltages is divided by 32 (thirty two) through the resistor string, wherein the step voltages for the first segment are selectively derived from the odd-numbered tap voltages, i.e. the first, the third, the fifth, . . . , the twenty-ninth tap voltages of the resistor string, while the step voltages for the second to eighth segments II to VIII are derived through combinations of the 31-th tap voltage and those extracted selectively from the even-numbered taps, i.e. the second, the fourth, the sixth, . . . , the 32-th taps. This system requires a large number of taps for the resistor string circuit. Further, selection has to be made to the odd-numbered tap or even-numbered tap in dependence on which segment of the .mu.-characteristic curve the sample voltage in concern belongs to. As the consequence, the structure of the local decoder becomes complicated and requires a chip of a large size for implementation in LSI circuit, giving rise to a problem.
As another type of PCM encoder which follows the .mu.-law with fidelity, there is a proposal in Japanese Laid-Open Patent Application No. 48472/1979, for example. In the case of the PCM encoder disclosed in this literature, a unique capacitor array is used which comprises a series connection of twelve first capacitors each having a first capacity, wherein both ends of the series connection are connected to the ground potential through capacitors each having a second capacity, while both ends of each of the first capacitors are connected to respective change-over switches through thirty capacitors of the second capacity so as to be selectively connected either to a reference voltage source or to the ground potential. The output voltage of the capacitor array is compared with a sampled value of a voice signal through a comparator. In response to the results of the comparison, change-over command signals for the switch are successively produced from a local decoder, thereby to obtain a PCM code corresponding to the sampled values. However, this PCM encoder is also disadvantageous in that the capacitor array is complicated, special arithmetic operations are required for the local decoder to derive the boundary values for the individual segments I, . . . , VIII, and so forth.